My invention relates to an improved data circuit for frequency modulating (or frequency shift keying) an oscillator, and particularly to such a circuit that permits the data to be capacitively coupled to modulate an oscillator with little adverse effect on the oscillator performance or on the direct current frequency control circuit for the oscillator.
Binary data is frequently transmitted over a wire or radio channel by shifting an oscillator frequency between two frequencies, one of which may represent a binary 1 or mark, and the other of which may represent a binary 0 or space. Typically, the transmitter oscillator frequency is shifted by applying one of two direct current voltages to a voltage sensitive frequency determining element, such as a voltage sensitive capacitor diode (or varactor). A first voltage causes the varactor to have a capacity that provides a first oscillator frequency. A second voltage causes the varactor to have a capacity that provides a second oscillator frequency. In most oscillators, a temperature compensating direct current voltage is also applied to the varactor to maintain the oscillator frequency relatively constant over a temperature range. In such oscillators, the voltage representing data must be capacitively coupled to the varactor so that the direct current temperature compensating voltage will not be disturbed or changed by the applied data signal voltages. However, such a capacitor presents problems. If it is too large, its relatively long time constant increases the time needed for the frequency transitions to reach equal steady state values or magnitudes. This delay can cause transmissions outside the allocated frequency channel. In addition, it may be desirable or necessary that the frequency of the carrier without applied data be returned to a center frequency midway between the frequency transistions representing data. The time constant of the coupling capacitor causes a delay in this return. If the capacitor is too small its relatively short time constant produces a differentiating effect which prevents clean and sharp frequency transitions of the oscillator. Various circuits and an optimum time constant have been tried, but as far as I am aware, none has produced a result that is as desirable as needed.